The present invention concerns a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a multispot telecommunication satellite. This type of satellite allows the use of several spot beams from antennas on board the satellite to cover contiguous geographical zones or cells, instead of a single broad spot beam.
Such multispot satellites allow several radiofrequency links to be established occupying the same frequency band on different spot beams.
In the case of a high bandwidth broadband satellite telecommunication system, the satellite is used in a bidirectional manner, i.e. at the same time for:
relaying data emitted by a gateway (linked to the ground network) towards a plurality of ground terminals: this first link of the point-multipoint type constitutes the forward link;
relaying towards the gateway the data emitted by the ground terminals: this second link, of the multipoint-point type, constitutes the return link.
It will be noted that a satellite broadcasting service can be considered as equivalent to the forward link of a bidirectional system such as described above.
An example of forward link in a multispot telecommunication network configuration is illustrated in FIG. 1.
Signals are sent towards a multispot satellite 3 on an uplink LM by a gateway 2 (also designated the central station) such as a ground gateway connected to an internet backbone 5. The gateway controls the network by the means of a management system of the network which allows the operator to supervise and monitor all the components of the network. The signals sent by the gateway are then processed at the level of the satellite 3 which amplifies them, derives them at a generally lower frequency then retransmits them from the satellite antenna or antennae on a downlink LD in the form of a plurality of spot beams forming elementary covering zones or cells C1 to C8 in which ground terminals 6 are situated. Each cell C1 to C8 is associated with a spot beam SP1 to SP8. It will be noted that in the case of configuration 1, the eight cells C1 to C8 associated respectively to the eight spot beams SP1 to SP8 form a group of cells served by the same gateway 2. In practice, the network 1 is formed by a plurality of gateways connected with each other via a ground network (Internet network for example). The return link of the ground terminals 6 towards the gateway 2 functions in an identical manner with an inverse direction of communication.
The coordination of the frequencies between operators is carried out within the framework of a regulation decreed by the International Union of Telecommunications (IUT): thus, by way of example, the Ka band for Region 1 (Europe, Africa, Middle East) is defined in Table 1 below:
TABLE 1ForwardUplink (of the gateway)27.5 GHz to 29.5 GHzlinkDownlink (towards the ground19.7 GHz to 20.2 GHzterminals)Return linkUplink (of the ground terminals)29.5 GHz to 30.0 GHzDownlink (towards the gateway)17.7 GHz to 19.7 GHz
It is observed that the spectra of the band Ka in uplink are adjacent (i.e. the intervals [27.5; 29.5] and [29.5; 30] present no discontinuity). The same applies to the spectra of band Ka in downlink (i.e. the intervals [17.7; 19.7] and [19.7; 20.2] present no discontinuity).
Given that the gain of an antenna is inversely proportional to the opening of the spot beam, a way of covering an extensive zone with a homogeneous and high gain is to use multispot antennae. For a given service zone, the greater the number of spot beams, the smaller the opening of each spot beam will be. Thus, the gain on each spot beam and hence the gain on the service zone to be covered will be increased. As we have mentioned above, a service zone to be covered is formed by a plurality of contiguous cells (elementary covering zones), a spot beam being associated with each cell; however, it is possible that a part of a service zone is disconnected from the others (an island, for example), and that the associated cell is disconnected from the other cells constituting the remainder of the service zone. A homogeneous multispot covering zone SA is represented in FIG. 2a), each cell being represented by a hexagon FH such that the covering zone is composed of a plurality of hexagons FH in which θcell is the external dimension of the cell expressed by the angle of the satellite associated with the covering. However, the antenna spot beam associated with each cell is not capable of producing a hexagonal shape, a good approximation consisting in considering a plurality of circular spot beams FC such as represented in FIG. 2b). The association of a spot beam with a cell is carried out taking into account the best performances of the satellite for said spot beam, in particular in terms of EIRP (Effective Isotropic Radiated Power) and of merit factor G/T (ratio gain over noise temperature): a cell is determined as the part of the service zone associated with the spot beam offering the highest gain on this zone from all the spot beams of the satellite.
Configuration 1, as represented in FIG. 1, uses a technique designated frequency re-use: this technique allows the same range of frequencies to be used several times in the same satellite system so as to increase the total capacity of the system without increasing the allocated bandwidth.
Frequency re-use schemes are known, designated as colour schemes, making a colour correspond to each of the spot beams of the satellite. These colour schemes are used to describe the allocation of a plurality of frequency bands to the spot beams of the satellite with a view to radiofrequency transmissions to be realized in each of these spot beams. In these schemes, each colour corresponds to one of these frequency bands.
In addition, these multispot satellites allow polarised transmissions to be emitted (and received): the polarisation can be linear (in this case the two directions of polarisation are respectively horizontal and vertical) or circular (in this case the two directions of polarisation are respectively circular left or circular right). It will be noted that in the example of FIG. 1, the uplink leaving the gateway 2 uses two polarisations with four channels for each polarisation, respectively Ch1 to Ch4 for the first polarisation and Ch5 to Ch8 for the second polarisation: the use of two polarisations allows the total number of gateways to be reduced. The eight channels Ch1 to Ch8, after processing by the payload of the satellite 3 will form the eight spot beams SP1 to SP8 (one channel being associated with one spot beam in this example).
According to a scheme with four colours (red, yellow, blue, green) with a frequency spectrum of 500 MHz for each polarisation, the transmissions being polarised in one of the two polarisation directions: circular right or circular left, each colour is associated with a band of 250 MHz and a polarisation direction. Within the framework of the invention, the use of a scheme with four colours is an example; any number of colours greater than three can be suitable; however, if one wishes to use to the best the isolation permitted by the use of the two polarisations, a number of colours which is a multiple of two is necessary.
In the whole of the following description, we will take the following convention:
the colour red is represented by hatched lines toward the right;
the colour yellow is represented by dense points;
the colour blue is represented by hatched lines toward the left;
the colour green is represented by dispersed points.
A colour is thus associated with each spot beam of the satellite (and hence a cell) so that the spot beams of a same “colour” are non-adjacent: the contiguous cells therefore correspond to different colours.
An example of a scheme with four colours for the coverage of Europe is represented in FIG. 3. In this case, 80 cells are necessary to cover Europe.
This type of scheme is equally applicable in uplink and in downlink. At the satellite level, the creation of a spot beam is made from a feedhorn radiating towards a reflector. A reflector can be associated with a colour so that a coverage with four colours is ensured by four reflectors. In other words, the generation of 16 spot beams of each gateway can be carried out via the use of four antennae (one per colour) each having a reflector, four feedhorns being associated with each reflector.
FIG. 4 illustrates a frequency plan broken down into an uplink frequency plan PMVA on the forward link, a downlink frequency plan PDVA on the forward link, an uplink frequency plan PMVR on the return link and a downlink frequency plan PDVR on the return link. The notations RHC and LHC designate respectively the right and left circular directions of polarisation.
The PMVA plan corresponding to the uplink on the forward link (of the gateway to the satellite) has 2 GHz (of 27.5 to 29.5 GHz) available frequency spectrum so that 16 channels of 250 MHz band pass are generated by a gateway (8 channels for each polarisation). These 16 channels, after processing by the payload of the satellite will form 16 spot beams. The hypothesis made here consists in considering that the entire spectrum of 2 GHz is used: it will be noted, however, that it is equally possible, in particular for operational reasons, to use only one part of the spectrum and to generate fewer channels. In the example above, 16 spot beams (and hence 16 cells) are generated from two signals multiplexing the 8 channels (a signal multiplexed by polarisation) generated by a gateway. Each multiplexed signal corresponding to a polarisation is then processed at the level of the transponder of the satellite so as to provide 8 spot beams; each of these eight spot beams is associated with a frequency interval from the two frequency intervals [19.7; 19.95] and [19.95; 20.2] and an RHC or LHC polarisation as represented on the downlink frequency plan PDVA.
The PDVR plan corresponding to the downlink on the return link (of the satellite to the gateway) has 2 GHz (of 17.7 to 19.7 GHz) available frequency spectrum so that 16 spot beams of 250 MHz band pass (associated with a frequency interval from the two frequency intervals [29.5; 29.75] and [29.75; 30] and an RHC or LHC polarisation as represented on the downlink frequency plan PMVR) issued from the cells are multiplexed at the level of the satellite in two signals (corresponding to each polarisation) to be returned towards the gateway (8 channels for each polarisation). We will still make the hypothesis that the whole of the spectrum of 2 GHz is used. Thus, in the case of Europe with a spectrum of 2 GHz used, one has a number of Nc cells equal to 80 and a number of active gateways NGWactive equal to 5 (namely the number 80 of cells divided by the number 16 of spot beams). It will be noted that if it may be that a part of the band is not usable, for example the part going from 17.7 to 18.45 GHz in the return link and the part going from 27.5 to 28.25 GHz in the forward link: in this case, the number of channels Ns per polarisation is equal to 5: consequently, the number of cells still being equal to 80 for Europe, the number of active gateways NGWactive becomes equal to 5. In any case, the number of gateways NGWactive is still greater than the number Nc of cells of the covering zone.
For the return link, each spot beam is associated with one of the following colours:
a colour red corresponding to a first band of 250 MHz (lower part of the available spectrum of 500 MHz) and to the circular right polarisation direction;
a colour yellow corresponding to the same first band of 250 MHz and to the circular left polarisation;
a colour blue corresponding to a second band of 250 MHz (upper part of the available spectrum of 500 MHz) and to the circular right polarisation direction;
a colour green corresponding to the same second band of 250 MHz and to the circular left polarisation direction.
The four adjacent spot beams of a same pattern are each associated with a different colour.
On the return link, the polarisations are inverted so that the colours red and yellow have a circular left polarisation and the colours blue and green have a circular right polarisation. The ground terminals emit and receive according to an inverse polarisation so that one can easily separate the uplink signals from the downlink signals: such a configuration allows less costly terminals to be used.
The payload of the satellite designates the part which allows it to fulfil the mission for which it was designed, i.e. for a telecommunication satellite 3 such as that shown in FIG. 1, to ensure the reception, processing (frequency conversion, filtering, amplification) and re-emission of the telecommunication signals issued from the gateway 2. The payload essentially includes the antennae of the satellite and the transponders (and not the equipment for control, propulsion or electrical power supply which belong to the platform of the satellite).
FIG. 5 shows in a known manner a functional block diagram of an architecture of payload 10 in forward link (from the gateways to the cells including the ground terminals) with multispot emission on the downlink.
After reception and selection of the polarisation, 2NGW multiplexed signals (in the example quoted above, NGW signals of 8 channels for each of the two polarisations) received from NGW gateways (or gateway) are each amplified by a LNA low noise amplifier 12. Each signal is then separated into Nc uplink channels by a signal divider device (demultiplexer) 13. The Nc uplink channels are then translated in frequency by a frequency converter circuit 14 generally formed by a local oscillator and are filtered by an input filter 15 (of the pass band filter type) so as to form Nc channels in accordance with the frequency plan of the downlink on the forward link (PDVA). The local oscillator is most often constituted by a voltage controlled quartz VCO (Voltage Controlled Oscillator) with a phase lock loop. The Nc translated frequency channels are amplified through a power amplifier 16 HPA (High Power Amplifier) generally formed by a channel amplifier 17 CAMP (Channel AMPlifier) and a travelling wave tube amplifier 18 TWTA forming Nc downlink spot beam signals. Each of the Nc spot beam signals is then filtered through an output pass band filter 19, and is then sent on a source 20 such as a radiating feedhorn towards a reflector for the formation of a spot beam. According to this configuration, the payload 10 includes:
2NGW low noise amplifiers 12 LNA;
2NGW signal divider devices 13;
Nc frequency converter circuits 14;
Nc input filters 15;
Nc high power amplifiers 16 HPA;
Nc output pass band filters 19.
FIG. 6 shows in a known manner a functional block diagram of an architecture of payload 100 in return link (from cells including the ground terminals to the gateways) with multispot emission on the uplink.
Nc signals received of Nc cells including the user terminals are each amplified by a LNA low noise amplifier 12. Each signal is then translated in frequency by a frequency converter circuit 114 generally formed by a local oscillator and filtered by an input filter 115 (of the pass band filter type) so as to form Nc channels in accordance with the downlink frequency plan on the return link (PDVR). As previously, the local oscillator is most often constituted by a voltage controlled quartz VCO with a phase lock loop. The channels intended for the same gateway (for the same polarisation) are then regrouped to form a multiplexed signal via a multiplexer 113 (with Nc inputs and 2NGW outputs): the structure of this multiplexed signal is identical to that of a signal emitted by a gateway towards the satellite on the uplink in forward link. One therefore has 2NGW output signals of the multiplexer 113. Each of the 2NGW signals is amplified through a power amplifier 116 HPA generally formed by a channel amplifier 117 CAMP and a travelling wave tube amplifier 118 TWTA forming 2NGW downlink signals in return link. Each of the 2NGW downlink signals in return link is then filtered through an output pass band filter 119, then sent on a source 120 such as a radiating feedhorn towards a reflector for the formation of the 2NGW signals with the destination of the NGW gateways. According to this configuration, the payload 100 includes:
Nc low noise amplifiers 12 LNA;
Nc frequency converter circuits 114;
Nc input filters 115;
a multiplexer device 113 with Nc inputs and 2NGW outputs;
2NGW power amplifiers 116 HPA;
2NGW output pass band filters 119.